With the rapid development of cloud computing, in order to save local storage resources as well as facilitate multisite access, more and more users choose to have their own data stored in cloud servers. For protecting data security, these users usually encrypt their private data before uploading them. According to the conventional encryption technologies, only a key holder can decrypt a ciphertext. Therefore, when users try to search cloud-saved outsourced data using a keyword, they must download the whole ciphertext before decrypting the ciphertext and performing the search locally. This obviously leads to huge network resource costs, local computation costs and storage costs. In order to assure security of cloud storage and provide safe and efficient data access at the same time, searchable encryption has been introduced. Due to the innate limitation of symmetric encryption, when applied in multi-user scenarios, searchable symmetric encryption usually requires a secure channel for transmission of secret information. For realizing safe communication and search in public networks, searchable public key encryption with keyword search (PEKS) has been proposed as a solution and extensively researched.
A typical PEKS application comprises plural transmitting parties, a server, and a receiving party. Any of the transmitting parties can use a public key of the receiving party to generate a searchable ciphertext for the keyword, and then send it to the server. The receiving party can use its own private key to generate a search trapdoor for the keyword, and send it to the server. After the server receives the search trapdoor, it can correctly find out all the ciphertexts containing that keyword, and return them to the receiving party. When it comes to searchable encryption, security and efficiency of the search have long been a concern of users and researchers.
In a recent research, Peng Xu et al. have introduced the concept of searchable public key ciphertexts with hidden structures (SPCHSs) (P. Xu, Q. Wu, W. Wang, W. Susilo, J. Domingo-Ferrer, and H. Jin, “Generating searchable public-key ciphertexts with hidden structures for fast keyword search,” IEEE Trans. Inf. Forensics Security, vol. 10, no. 9, pp. 1993-2006, September 2015.), wherein all the searchable ciphertexts of the same keyword are organized using an invisible chain, thereby linearizing search complexity. While their instantiated scheme does improve search complexity, the complicated cryptographic operation of pairing during search is linearly dependent to search results, and therefore leaves considerable room for improvement in terms of search efficiency.
This makes how to achieve high-speed search a pressing need to be addressed for designing a practical searchable public-key encryption scheme.
In addition, tremendous development of Industrial Internet of Things (IIoT) has been fueled by Industry 4.0, and wireless sensor networks (WSNs), particularly cloud-assisted wireless sensor networks (CWSNs), as an essential part of IIoT, has been applied into more and more areas, like healthcare, agriculture technology, national defense, environmental monitoring and smart conferencing. Since the data collected by sensors are usually of high sensitivity, data privacy protection has become a hot topic in the field of CWSNs.
Many encryption methods have been applied to CSWNs by researchers, such as homomorphic encryption, attribute-based encryption, hybrid encryption, symmetric encryption and so on. These cryptography schemes are focused on safe data transmission and storage in CSWNs. As to secure data search, searchable public key encryption with keyword search (PEKS) is a recognized solution. However, the existing PEKS schemes seem less efficient when it comes to generation of searchable ciphertexts and search of ciphertext data. In a CSWN model, data are encrypted by sensors. Considering that such sensors have limitations regarding power and calculation capability, we do need a lightweight encryption algorithm to reduce sensor costs.